The present invention relates to high-speed data transmission and in particular to signal structures for double side band quadrature carrier (DSB-QC) modulation.
In U.S. Pat. No. 3,887,768 issued June 3, 1975 to Formey, Jr., et al for SIGNAL STRUCTURES FOR DOUBLE SIDE BAND QUADRATURE CARRIER MODULATION the inherent advantages of DSB-QC over single-sideband (SSB) and vestigial-sideband (VSB) are discussed in detail. Briefly, DSB-QC systems can be designed to have a much greater insensitivity to phase jitter on the line, or to phase error in the recovered carrier than SSB or VSB signals while permitting a coherent local demodulation carrier to be derived from the received data without requiring transmission of a carrier or pilot tone.
The previously mentioned U.S. Pat. No. 3,887,768 describes a DSB-QC modulation system in which the signal points are mapped in the complex plane on concentric rings the signal points of which are rotated by 45.degree. from those of the next adjacent ring. While the disclosed DSB-QC constellations combat the combined effects of noise and phase jitter as discussed in the reference, in fact, improvements in the state-of-the art carrier equipment has itself contributed substantially to the reduction of phase jitter on many communication channels so that signal constellations designed to provide the best compromise performance between noise and phase jitter are no longer optimum in the sense of overall performance, wherein the "best" performance is defined as lowest overall bit error rate.
In order to attain higher data rates in a given bandwidth, higher signal-to-noise ratios in the communications media are required. As higher signal-to-noise ratios are required, constellations for more signal-to-noise efficient signals are necessary. In the presence of noise alone, signal constellations with points equally spaced on a square grid provide a near optimum performance. Moreover, such a pattern permits simple encoding at the transmitter and simple decoding or detection at the receiver. It is known that for a given error rate and bandwidth a square grid constellation offers better signal/noise performance than a comparable concentric ring type constellation. In fact, for example, a well known ring type constellation employing 16 points for 9600 bit transmission in a Nyquist bandwidth of 2400 Hz requires 1.3 dB greater signal/noise ratio for a given symbol error rate than a comparable square grid constellation. The well known concentric ring constellation discussed above is that proposed by CCITT Recommendation V.29 (offered commercially by Paradyne Corporation of Largo Florida as its MP-96 Data Modem). The square grid constellation is employed by Bell System in their model 209 Data Set.
The sacrifices paid for the greater signal to noise ratio of the square grid pattern over the concentric ring pattern are that:
1. The number of useable points must equal 2.sup.M where M is an integer and thus M can only equal 2, 4, 8, 16 etc. As a result a grid such as 3.times.3 could not be used.
2. As the number of points increases the distance from the origin of the furthest point relative to the root mean square distance increases rapidly. Since the distance from the origin is proportioned to the voltage necessary to generate the point, the peak to average voltage ratio becomes large and may lead to clipping in most communication media.
In view of the above, it is the principal object of the present invention to provide improved DSB-QC signal structures developed to provide near optimum performance in the presence of noise.
A further object is to provide such signal structures which allow simple encoding and decoding or detection.
A still further object is to provide such signal structures wherein the points in each of the four quadrants may be differentially phase encoded such that an absolute carrier reference is not necessary.
Other objects and advantages will be self evident from the description of the preferred embodiments of my invention.